[ARC5] Transmiitters: Parameters?

[Howard Holden]

Hello Ken, finally got around to measuring my unpainted BC-458A. It was 
modified by a previous owner with an SO-239, relays removed, and power plug 
changed but no changes to the main works.
Results as follows:

Ep 490V
Ip 160 mA
Es 280V
Is 17 mA
Eg -60V
Ig 4 mA
P in 78W
P out 45W
Osc Ep 210V Ip 20 mA not regulated, through a 1600 ohm series resistor from 
245V center tap of amp B+.

These voltages are consistent with a homebrew pair of 807s I run with the 
same supply.

Screens are fed through a 12.5K dropping resistor from plate voltage. Grid 
resistor measures about 16K.

I let osc run continuously, about 1.5Khz above output freq with key down. 
Final cathodes keyed.
On initial key down osc drifts about 100 Hz up, then stabilizes after about 
2 seconds. No noticeable chirp
from the higher unloaded osc freq  despite no regulation, but a few Hz drift 
through a transmission.

I use an external 430 pF cap (cap and coax connecting cable)  across the 
output, with no series L from the transmitter to feed approx 50 ohm loads.
Tried the series cap instead, and it's not nearly as good as the parallel 
cap, and harder to work mechanically.

Also working on trying to get an ATA for 80M to sound as good as this 458 
does.

73, Howie WB2AWQ



-----Original Message----- 
From: Kenneth G. Gordon
Sent: Monday, March 21, 2016 7:46 PM
To: ARC-5 List
Subject: [ARC5] Transmiitters: Parameters?

For those of you who are actively using your ARC-5 transmitters on the air, 
I
am very curious about what operating parameters you are coming up with:
plate voltage and current, grid voltage and current, screen voltage (and
maybe current), power input, power output, etc.

I am especially interested in what you are seeing for grid voltage and 
current.

I'd also like to know what value of screen-voltage dropping resistor you are
using (if any), what source of voltage for the screens you are using, and 
what
the measured screen voltage turns out to be.

I would love to get one of my junkers in operation here, mainly just to see
what it is going to sound like on the air.

FYI, according to the RCA Transmitting Tube manual, in Class C as an
amplifier, with 600 VDC on the plate, the grid voltage SHOULD be -45 at 4.0
mA. This is with an 11.2 kOhm grid resistor, but the grid-resistor in the
ARC-5 transmitters is 15K, which at 4 mA would give a grid voltage of -60,
which would then require more drive.

I am thinking that the grid-current is low in the ARC-5, possibly well below 
4
mA, since the driver is the 1626 and the grid resistor is high. This would
make for a pretty inefficient amp and in addition, could contribute to 
chirp.

Ken W7EKB
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